text:
Why can gliders fly for so long?
When you throw a paper airplane, it flies for a couple of seconds before losing speed and stalling. With a glider, it has a very long wingspan, but you take off with another plane or winch tow pulling it and then it is released on its own. Shouldn't the glider lose all speed and stall?

Try throwing a well-made and aerodynamically optimized paper airplane from a couple thousand feet in the air.

You mention the glider's long wingspan, but don't ask anything about it. Do you think that is related somehow?

Good gliding paper airplanes can get 15+ seconds if hand launched on level ground.  Not every paper airplane is a dart build for speed and distance that falls out of the air after a second or three.

The glider doesn't even have to be anywhere near as fine as the one pictured. When I was learning to fly gliders one of our instructors kept an old Schweitzer-222 aloft on an unremarkable day for more than three hours over Waukegan, IL, thereby cheating some of his students (the ones on the ground) of what we thought of as our rightful flying time ;-). If the pilot is good, he can find thermals where mere mortals cannot.

You're making your paper airplane wrong if it stalls. It means your paper airplane is too tail-heavy. My paper airplanes tend to maintain speed until they reach the ground. The key to it is CG adjustment. In order to retain energy your glider needs to fall nose-first. Falling nose-first allows the wings to move forward through the air (instead of downwards). Moving forwards allows the wings to generate lift (instead of merely drag). Lift delays falling to the ground. If your paper airplane lose speed and stall after a few seconds, add one (or several) paperclips to the nose.

@slebetman its probably cause I didnt throw it downwards. And also my paper airplane has pretty small wings. Though the wings have a slope on the top of the wing which gives it great lift. Does some pretty cool tricks also. Do you know any good paper airplane designs that I could build.

@Ethan: Get a copy of The Great International Paper Airplane Book if you can. It's old but the designs are timeless. I spent many many hours with my copy of that, and learned a lot.

I've seen video of soaring in a Taylorcraft (similar to a Piper Cub) with the engine shut down.  Not even built as a motor glider, never mind a sailplane.

@ZeissIkon - nice, I did that in a Ces 152 once for about 45 min, but was in wave.

@quietflyer The Taylorcraft was in thermal lift -- and didn't have electric start, so once the pilot shut down the engine, they were going to either thermal a good bit or land out.

A glider and a paper airplane operate on the same principle: Exchanging whatever potential energy (altitude) they have for the kinetic energy (airspeed) required to keep air moving over the wings so they produce lift, giving a stable, controlled descent (glide).
In both cases the airfoil will not stall unless it exceeds its critical angle of attack which is independent of the airspeed. You can continue to fly (with the wing not stalled and producing lift) as long as you still have altitude to trade for that airspeed, and if you do it right you will eventually run out of both altitude and airspeed fairly close to the ground, where you can have a soft landing at a speed where the aircraft is not damaged. In the case of a manned glider hopefully this happens on a runway where you can get another tow plane to take you up again.

Both aircraft can extend their glide time by finding other sources of  lift - thermals, ridge lift, etc. - to carry the vehicle to a higher altitude, providing more potential energy.  Gliders are better at this for one major reason: They have a pilot who can keep the aircraft in an area of rising air longer to gain the most altitude possible.
A paper aircraft can similarly be made to fly for a longer period of time if given an adequate source of lift (equal to or exceeding its minimum sink rate). There's a rather famous video of the "infinity paper plane" which is a paper airplane circling over an electric stovetop, but I'll spoil it for you and tell you that one is a hoax (the general consensus is the plane was on a string).
This one, however, can be replicated fairly easily: The result is more similar to a helicopter in autorotation or an autogyro in a glide than a traditional fixed-wing glider, but it's a good demonstration of the principles involved.

If you've got proof of the paper-airplane hoax, there's a question over on Skeptics.se that could use an answer.

Once I lost a paper airplane to a rising current of air, it ascended until it was completely out of sight.

@Mark It's agreed in the comments that this one is an obvious hoax. For one, he switches the cookers off, not not, and they are off for the whole experiment.

@yo' Having the electric elements turned off doesn't mean they are not still putting out a large amount of heat. It takes quite a while for an electric element that was on a high setting to fully cool off to room temperature again when turned off.

@ToddWilcox Certainly then, the described video is not an infinite flight.

@Mark No proof, but it's generally accepted that it's "too perfect" and I don't know of anyone who has repeated the results. The one I linked to however is pretty easy to reproduce. (In neither case is it really "infinite flight" as eventually some air current will come along and disturb the system by blowing the aircraft out of the thermal column, but with the second one you can probably keep it going longer since it's a controlled equilibrium trying to fall near-vertically.)

The range in the video looks like an induction cooktop to me... in which case without any pans there'd be no heat at all!

@2012rcampion: Looks like, but isn't. In fact, that's a fairly normal glass-ceramic cooktop: see the heating spirals below.

The primary difference between a glider and a paper airplane is that most folded paper airplanes have very bad aerodynamics.
Most folded paper airplanes have short stubby wings. This in itself makes it very draggy. The ideal wing with absolute minimum drag has a wingspan of infinity. Obviously it's not possible to build that ideal wing (we don't have infinite number of atoms in the universe). But generally, all else being equal, the longer the wingspan the less the drag.
Secondly, paper airplanes have very poor airfoils compared to real gliders. They're not even flat plates (which would actually be a fairly good airfoil at the Reynolds Number they fly). The folds typically add uneven thickness to the airfoil making it a bit wedge shaped. This adds a lot of drag.
Finally, you mention that the paper airplanes you're used to flying tend to lose speed and stall. Stalling definitely reduces glide time. Stalling wastes energy. A smooth glide will tend to stay aloft longer. A real glider is trimmed to glide smoothly and not stall.
Toy gliders need not suffer the weaknesses of paper airplanes. A good toy glider should be able to fly for at least 10 seconds per flight on average (or at least 3 seconds if you throw it level instead of up). Really well designed toy gliders can fly for more than 20 seconds and competition gliders should be able to fly for around 1 minute.
If you've never seen a balsa glider fly before you'd think that it's flying for a very long time. Here's one example (it was the video that introduced me to hand-launched gliders):

. And here's a more recent example:

The world record for hand-launched gliders is almost 2 minutes:

These gliders fly longer than the typical paper airplane because they're well designed and are aerodynamically more efficient. Note that most of these balsa gliders (including the world record glider) have eyeballed, imperfect airfoils. Sometimes they're even triangular or just have the leading and trailing edges beveled. So the airfoil is not as important as the design, trim and balance (CG) of the plane.
Paper airplanes need not suck. Here's a clip of the world record (duration) flight of a paper airplane:

. If you google around for the design of that plane you'll find that it has a lot of nose weight (good balance/CG) and bent up trailing edges (good trim).
So the key lessons are:

Balance your plane (adjust the CG). This is true for paper airplanes, RC planes and even real airplanes of all sizes. Bad CG is the number one reason planes stall. There are formulas and rules of thumb to calculate the correct CG for your plane but for a paper airplane I'd suggest just experimenting with adding and removing nose weight (that's what people do with balsa gliders).
Reduce drag. For paper airplanes you can try creasing the fold better so that the folded part is as thin as possible. Gliders can increase wingspan and choose low drag airfoils to reduce drag but that's not quite applicable to paper airplanes. Then again, there are paper gliders that have relatively long wingspans (compared to typical paper airplanes) that can glide very well:

. But that's entering into the scissors and tape category.

The glider and a paper aircraft are designed and operated in different ways which affects their stalling characteristics.

Both aircraft trade one form of energy (potential energy or altitude) in order to gain another (Kinetic energy or forward speed). In general, the gliders are in (a controlled) descent unless they encounter thermals or rising air. This is what causes their gliding. The same happens with any unpowered aircraft; however, the gliders have a lot of altitude to expend compared to the paper aircraft.
The gliders are designed to be highly aerodynamically efficient, with extremely high lift to drag ratios, usual in the range of 50:1 or more. That means that they can glide for extremely long distances, left to their own devices. The paper airplanes are not like this. However, with proper design, they can be made to fly longer.
The gliders are usually flown by (skilled) pilots, who know how to find and make use of the aforementioned thermals, upward air currents etc, which helps in increasing altitude and keeping the aircraft in air longer.
Most of the gliders are extremely stable compared to the paper aircraft (which usually has only a wing and no stabilizer), which prevents them from stalling when left alone. In fact, in some gliders, the best way to recover control is to let go of the controls. It basically means that the glider returns to level flight when no control inputs are given.

In order to keep an aircraft flying, you need to keep adding energy. In an aircraft with an engine, the energy is primarily in the form of fuel.
In a glider, the additional energy comes from the lifting action of rising air. Glider pilots will find areas of rising air (they have sensitive instruments to assist with this), and circle in that area to increase altitude. Once they have enough altitude, they will fly in a more or less straight line toward their destination, stopping along the way to circle again and gain more altitude.
Rising air may come from "thermals" (warm air bubbling up through the atmosphere), "ridge lift" (air rising up a slope), "wave lift" (produced by mountains), and others. You can read more about lift at Lift (soaring). There are different flying techniques required to best take advantage of each kind of lift.

You compare 2 different things, the paper plane and the glider. Gliders take weather into advantage to increase their altitude and sustain flight. Paper planes do not experience the same weather phenomena as gliders do. And of course they lack pilot and control surfaces.
Wikipedia mentions 4 ways to gain energy:

Thermals (where air rises due to heat),
Ridge lift, where air is forced upwards by a slope,
Wave lift, where a mountain produces a standing wave,
Convergence, where two air masses meet

Many people mentioned the difference in the shape of the wings and airfoils. But even if you just simply downscale a real glider to a 5 cm wingspan it won't fly as well as the big one.
The Reynolds number for the small scale plane is very small and the viscous drag becomes large. Turbulence will not be strong enough to prevent flow separation at angles of attack which are still OK for the real plane, because the laminar boundary layer is more susceptible to separation. The separation strongly reduces the generated lift and may lead to stall.
It doesn't mean your paper plane will always stall, but you must be careful with the angle of attack and play with the centre of gravity.
Small models have to use special low Reynolds number airfoils, but will never be as efficient as larger gliders, because the skin friction drag is so important. Even typical radio controlled glider models are usually much larger than typical paper planes.

From the moment the tow or winch is released, a glider starts descending.  If the atmosphere were completely still, apart from trading speed for height, it will soon have to land.
Gliders are designed with very high lift/drag ratios.  40:1, 50:1 even 60:1 or higher.  They stay aloft for long periods because of this and their ability to take advantage of, and the pilots skill in finding, thermals and upward air currents caused by turbulence off ridges, mountains etc.
Paper aircraft have a completely different design (you could not build a glider like the one you show from paper) and can fly for a lot longer than a couple of seconds.

One way or another, a flying object has to push down on the air to keep flying.  Given that air is fluid, we have to make the change in momentum of the air at least as big as the weight of the aircraft. Now:

force = difference in (mass * velocity)

But the change in energy required to do this alone goes as

change in energy = change in (mass * velocity squared)

So for efficient flight, we want to push down on a large mass of air with a small change in vertical velocity, rather than a small mass of air with a large velocity.
Gliders, with their long skinny wings, are optimized to do this: they fly relatively slowly intercepting a relatively large mass of air and pushing down on it gently.
At the other extreme, vertical take-off jets, like the Harrier, have to take the opposite approach. They need to fly fast, so in hover they push down a relatively small mass of air very quickly.  This is horribly inefficient so a Harrier can not hover for very long before its fuel is exhausted.
In addition to this, a glider is relatively light as it does not have to carry fuel or an engine.  It also is made a smooth an slippery as possible to reduce air resistance.

Basically, the Aerodynamic shape and wing shape along with the power of lift working together to keep the glider up for long periods of time.

Have to put more details in your answer for it to be accepted by the user who asked the question.

Other people have already mentioned the use of weather to keep the glider afloat longer.
I think you are asking, "How do wings work?"
Aircraft wings are modeled after what we have seen in nature. The basic shape is the wing is flat on the bottom and curved on the top. This curve is similar to a curve found on part of an egg with the bulk of the width located in front of the wing. (You can google up more optimized shapes.)
Forward motion will compress the air above the leading portion of the wing but will also create some drag in the process. The following portion of the wing slowly thins to the trailing edge. This portion of the wing is the area that has the negative air space (lift). It is this negative space that all aircraft rely on. Helicopter blades and engine propellers also make use of this basic shape.
A gliders initial propulsion is a tow, but after that they rely on gravity for propulsion and the lift created by the wing shape, in addition to weather assists.
Comparatively, [without using specialized measurement tools or going into a higher math explanation, the flatness of] a paper airplane has [only negligible] lift [when comparing to an optimized man made wing or wing found in nature] and relies solely on you for a one time propulsion [unless you are lucky enough to find an exhaust grate or the like to fly over].

a paper airplane has no lift this is wrong.

Paper airplanes do in fact generate lift, but loose thrust and stall after a few seconds. It may seem like it but its not.

Anything that does not accelerate towards the ground at the speed of gravity when moving horizontally through air does so by generating lift, a paper plane does not fall straight to the ground when flying so it does have some lift

The reason a Glider can stay up is for the simple reason that the pilot is there to read the weather and the terrain, and to coax the aircraft along areas and lines of lift.


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How does the cabin pressure controller work in a large commercial airliner?
The cabin pressure controller controls the pressure in the cabin in relation with aircraft altitude, and a manual control is possible using the overhead panel:
A320

Air Conditioning and Cabin Pressure panels on an Airbus A320 (source: Olivier Cleynen on Wikimedia)
B737

Pressurization panel on a Boeing 737 NG (source: B737.org.uk)
Overall how do the controller and the related equipment work in a commercial airliner, e.g:

What is the source used for pressure reference?
How outflow and relief valves and packs are related to the controller?
When and how is used the manual control of the pressure?

The System you are referring to is described in ATA chapter 21. I will report here an example for A320
An overview of the system is reported below (from the ATA21 report):

What is the source of pressure reference?

In flight the outside pressure signal can be come from either:

The Air Data Reference System (ADIRS), for the pressure altitude.
The Flight Management and Guidance Computer (FMGC), for the landing elevation.

If FMGC not available, uses the landing elevation from the ADIRS and the landing elevation

How the different valves (outflow, relief) and the packs are related to the CPC?

There are two independent safety valves controlled by two independent Cabin Pressure Controllers (CPC) and actuated by three independent motors.
 The pressure in the cabin is controlled only with these valves, while the temperature of the cabin is controlled by mixing the air from the packs with the one of the cabin. In particular the valves are opened and closed to keep the desired pressure profile according to the logic dictated by the different flight phase. .
Different constraint drive the controller:

Cabin pressure at cruise altitudes is limited to an equivalent ISA pressure of 8000ft, due to structural fuselage requirements. By providing higher pressures levels (equivalent to lower ISA equivalent altitudes, e.g.: sea level) the fuselage would overstrain. A (very) simplified example  of this is a balloon in a vacuum chamber.
Maximum descent rate is kept under 750 ft/min for passenger comfort. Otherwise your internal ear starts hurting.
At take-off on ground the cabin is de-pressurized till -0.1 psi with a max rate of 400ft/min to avoid pressure surge at rotation

When and how is used the manual control of the pressure?

Manual control of the pressure is used when both the CBC controllers fail or when the crew uses the cabin press control panel, that you showed. In case of aircraft ditching the inlets are closed as well with a ditching button (first image, right in the CABIN PRESS panel)

Interesting answer. I discovered the existence of ATA chapters, thanks a lot!

This answer deserves a little more work. Obviously the cabin pressure goes much higher than the pressure of the ambient atmosphere at 8000'. Also obviously a hgher cabin pressure altitude would decrease not increase the strain on the aircraft. Did you mean to say that the difference between cabin pressure and outside pressure is kept to 8.06 psi maximum, or the cabin would explode like balloon?

Also, can you please say more about this business of pressurizing to below field altitude before takeoff, "to avoid a pressure surge at rotation"? And is the same thing happening before landing? I don't get it, seems to just inflict unnecessary pressure changes on the passengers' ears.  Also did you mean to say before take-off on ground the cabin is pressurized (not de-pressurized) till -0.1 psi with a max rate of 400ft/min to avoid pressure surge at rotation? And can you please explain what would cause a pressure surge at rotation?

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For the elliptical wing, which property is actually elliptically distributed?
Elliptical wings are sometimes used:

One of the 50 Spitfire still flying in the World. Source
I read it's because this shape allows a perfect distribution of aerodynamic forces. But actually:

What is actually elliptical in the elliptical wing?
What is regularly distributed: Is it lift? or drag? or something else?

It is both the planform and the circulation distribution. Note that circulation is not lift coefficient but bound vortex intensity. You can interpret it as local lift coefficient times local chord.
On the untwisted elliptical wing the local lift coefficient is constant over span, and changes in angle of attack over the linear range will change the lift coefficient equally everywhere. This, when combined with the elliptical chord distribution over span, means that the circulation distribution will stay elliptical over the linear angle of attack range. This is the special characteristic of an elliptical wing: While any wing can have an elliptical circulation distribution at one angle of attack (given the right twist distribution), the elliptical wing will keep that elliptical circulation distribution over the whole operating range.
With an elliptic circulation distribution comes also a constant induced angle of attack and downwash angle over span. I guess this is expressed by some authors with the term "regular".
However, only the aerodynamicists will see that as an advantage. Both weight and stall characteristics of elliptical wings are less than optimum; the low induced drag coefficient is bought with higher structural mass and, consequently, lift. A more triangular circulation distribution will yield the lowest wing weight and overall drag for a given non-lifting mass (that is, all mass that is not involved in lift generation, especially the payload). Note that for such a triangular distribution drag will be highest near the center.
When people talk about elliptical lift distribution, they mean lift per span. I prefer to use the more correct term circulation, since lift is a force as in pressure times area and can only be produced by a whole wing or at least wing section, not one spanwise station.

(Edited after comments by mods. Thanks for guiding me through the process!)
The only thing that is elliptical is the planform shape of an elliptical wing.
Many people unfortunately confuse an elliptical pressure distribution with the
pressure distribution over an elliptical wing. They are not equivalent.
On an untwisted elliptical wing the local lift coefficient is not constant over
span, as some here and in many other places on the internet maintain.
Arguments from lifting line theory are not appropriate for discussing the
behaviour of the flow near wing tips. It is unreasonable to expect lifting line
theory to be valid near the wing tips because is not a consistent large aspect
ratio asymptotic expansion.
See: Van Dyke, "Perturbation methods in fluid mechanics", 1964.
The span loading includes a logarithmic term, hence it is not elliptical.
Furthermore, it cannot induce a constant downwash, and so at the trailing edge
the vortex wake does not start as a flat sheet.
The process whereby the vortex sheet rolls up is far more vigorous than if it
started out as a flat sheet because the relatively weak effect of viscosity
would be the main mechanism driving that rolling-up process. There is a strong
upwash in the flow-field near to and around the wing tips which initiates the
rolling-up process far more vigorously.
See, for example, among many other papers:
Peter F. Jordan, "Exact Solutions for Lifting Surfaces", AIAA Journal, Vol. 11, No. 8, 1973., pp. 1123-1129.
Peter F. Jordan, "On Lifting Wings with Parabolic Tips", ZAMM 54, pp. 463-477, 1974.

Hello Lysistrata, welcome to Aviation.stackexchange.com. I think your answer has great potential, but currently it is only a comment to Peter Kämpf's answer. Can you edit your post and answer the original question, thereby addressing where you think Peter's answer falls short? Otherwise it may be flagged as "comment to other post" and subsequently deleted.

My comment is a criticism of Peter's answer, specifically regarding constant spanwise lift coefficient. (I have no problem with the other parts of his thoughtful answer.)
I gave reasons for why I believe that part of his answer is incorrect, and I have cited appropriate references. I'm not sure that I can do much more than that.

Welcome too, I have to agree with DeltaLima. It's not that your comment is bad, it's great, but the site rules don't allow this format. Please answer the original question in your post as well, to prevent your post from being deleted. It's just to make sure your post fits the community rules, because the community on stack-exchange is strict.

@Lysistrata I understand you want to criticise Peter's answer, and I think that is fine. However, this is not a forum, it is a question & answer site. Each posted answer should in fact answer the question. Comments can be made under an answer to ask for clarification or to criticise. Since your comment is too big to be left in the comment field, the site's format causes a problem here. I am pretty sure that the community will (and I agree with that approach) vote to delete answers that are in fact comments, just to prevent this site to develop into a discussion forum.

For a moment I thought you deleted your answer and all the good references were gone. Thank you for editing your answer and putting it back in! By the way, we (@NoahKrasser and me) are not mods, we're just ordinary users like you. It's the combined effort of the community that makes this site great.

The beautiful shape of the Spitfire planform also made it difficult and expensive to produce. The target was an elliptical lift distribution, at the time of design regarded as resulting in the highest lift/drag ratio.

This lift distribution is obtained by shaping the wing geometry, in the case of the Spitfire by making the chord an elliptical function of wing span. The other way of achieving this is to use a combination of wing taper and wing twist: the chord reduces linearly with span, plus the wing is twisted with the wing tip having a lower angle of attack than the wing root.

This shape creates elliptical lift distribution as well, but at only one AoA of the aircraft. The elliptical wing has an elliptical planform and elliptical lift distribution throughout all aircraft AoA.
The eliptical planform wing has a constant AoA, and drag changes as a function of wing chord only. The twisted wing has a wing root with a higher AoA, and a wing tip with lower AoA than aircraft AoA: the fat bit of the wing has the higher drag, since wing drag is a function of chord and AoA. So total wing drag of the tapered wing is higher than that of the elliptical wing.
The tapered twisted wing rules though, because:

It is cheaper and faster to produce.
The wing tip stalls last, and that is where the ailerons are.

Actually, the reason for the elliptic shape was to house the guns in the thinnest possible wing. Mitchell was cited by Shemstone as follows: "I remember once discussing the wing shape with him and he commented: “! don’t give a b..... whether it’s elliptical or not, so long as it covers the guns!”

This answer is quite wrong, it wholly mis-characterises the Spitfire's wing. It was twisted, with (I think) around 3 deg of washout. The aerofoil changed along the span, being thinner and less cambered at the tip. In consequence its lift distribution was closer to the structurally optimal bell-shape than the elliptical. This was of course deliberate, in order to combine superb handling qualities with high efficiency. As @PeterKämpf says, the elliptical shape was an accident of the specification change from four guns to eight.

@GuyInchbald I think you are too strict. The archetype for the Spitfire wing was the Heinkel 70 wing, and here the elliptic shape was chosen to have enough internal space for the retractable landing gear. Both camber and thickness were reduced towards the tip as well, so Mitchell simply stuck to what the Günther brothers had chosen. But since the Günther brothers were obsessed by optimum aerodynamic shaping, the elliptic result wasn't entirely unwelcome, either.

@PeterKämpf That He 70 ripoff meme is another myth peddled by too many mainstream writers who fail to check their sources; the designs were quite independent. Any resemblances are convergent evolution not copying. The He 70 was designed with fixed u/c and the retraction mechanism introduced later; the Spit wing was evolved from a straight taper to fit more guns within its chord. It was not Mitchell who designed the Spitfire wing, it was Shenstone and another colleague whose name slips my mind at the moment. Even one error from you is unusual, three in one comment is remarkable. Take care.

@GuyInchbald He who sits in a glass house should not throw stones. The fixed gear design was the Heinkel 65. The He-70 had a retractable gear from the start.

@PeterKämpf Ha, you are right. Touché! Nevertheless we may think of the He 70 as an evolution of the 65, just as the Spitfire was an evolution of the initial Type 200 design.

And anyway it was the Type 300. I'll get my coat.

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What is a Cruise Captain?
I wonder what is a Cruise Captain, albeit I assume they are here to fly the cruise phase.

Differences in responsibility / authority with a Captain?
From training / certificate standpoint, what is required?
When are they seen on board?
Is the Cruise Captain the pilot-in-command when they officiate?

Can you provide some context to the term, perhaps a link to where you saw the term (an article, movie, book etc.).

@Dave: In this ARAIC MD-11F incident report (§2.1): "A total of five persons [...] of which the three were operating crewmembers consisted of a Pilot In Command (PIC), a Co-Pilot who was qualified as PIC (hereinafter called the Second In Command (SIC), and a Cruise Captain as a relief crew [...])".

According to the discussion here it seems a "cruise captain" is the captain who flies only during the cruise stage of flight. You can find a job listing for a "cruise captain" here but it does not elaborate much on the duties. To act as the pilot in command (at least by FAA regs) they would have to be current, and type certified in the aircraft in question. It seems the term is more a turn of phrase and the regulations are not any different for a cruise captain. On very long haul flights that may require 3 full crews, there will be a crew in the middle section of the flight that never sees takeoff or landing from the cockpit.
The report you have linked in the comments defines cruise captain in the foot note on page 3.

Cruise Captain in this report refers to a pilot certificated as cruise
  captain by the Malaysian Aviation Authority, and is qualified by the
  company to act as PIC during cruise only and who was engaged in the
  operation of the aircraft.

It sounds like "Cruise Captain" in this case is a Malaysian Aviation Authority term (although it may apply elsewhere as well). I am looking for the actual regulation that defines it but have not yet come across them.

On long haul flights that require additional crew members, airlines use various ways to meet the regulatory or contract requirements.
Many airlines add just additional First Officers (co-pilots). For some airlines this is not desirable because there is no Captain in command while the Captain takes his break.
Some airlines will add a second Captain on these long haul flights while a few have created a new position they call "Cruise Captain". This is usually a senior First Officer who has additional training and responsibility to satisfy the need to have a "Captain" on the flight deck for all phases of flight without actually paying for full Captain.
Some airlines have also created a junior position called "Cruise Relief Pilot" who is paid less than a regular First Officer and can only operate during the cruise portion of the flight.

A pilot which only operates during the cruise portion of the flight is......kinda boring, if I might say.

@kevin My goal is to make every flight as boring as possible.  Exciting flights tend to be fatal.

"Many airlines add just additional First Officers" Do any airlines actually do this? This would only allow the First Officer to take a break, not the Captain who would be forced to fly the entire flight with no (extended) break. It's not just about having a Captain in command, it's about sitting in the left seat. My understanding is that only someone trained to sit in the left seat is allowed to do so, which a normal First Officer is not. If a First Officer is trained for left seat then they have additional certification as a relief pilot, they are no longer just a First Officer.

@StevePemberton yes many airlines do it. AF447 is perhaps an argument against it, but a crew of 1 captain and 2-3 first officers (or a second officer) is very common and happens without incident every day. The laws don't require the captain to physically be in the flight deck at all times.

@Ben - but do they sit in the left seat? I realize that the left seat doesn't have to be occupied at all times, but my understanding is that no one can sit there without being trained for it. I think a First Officer with training for left seat is no longer "just" a First Officer, although the actual terminology for what they are classified as might vary per country/airline. Maybe they are colloquially referred to as First Officers simply because they have not had Captain training, only First Officer training with additional left seat training.

@StevePemberton yes they do sit in the left seat, that's better than it remaining empty. I can't answer with authority but I think left seat training is only required for takeoff and landing. But left seat training doesn't mean a promotion above first officer. I personally have photos sent from a second officer from the left seat in cruise.

@Ben - I find it unlikely that this would be allowed even in cruise. In an emergency situation where they suddenly have to at a minimum operate switches, and possibly even fly the plane, it would be extremely dangerous to be sitting there without having done some simulator time in the left seat reacting to emergencies, etc. and getting checked out for operation in the left seat. Although I suppose after enough time sitting there they would eventually gain those reaction skills through osmosis, I don't think that's how the regulatory agencies would view it.

There was a time when crews had 2 Captains (rated and trained on the left seat and 2 First Officers (rated and trained on the right seat). Companies have made measures to cut down on costs in salaries so instead of the usual crewing, they've made it 1 Capt (rated and trained on the left seat), 1 FO (rated and trained on both right and left seat) and 2 Second Officers (rated and trained primarily to fly during cruising phase on the right seat, hence the derivation of the term Cruise Relief Officer or in the case of the First Officer, Cruise Captain. Differences in policies and protocols dictate responsibilities and duties

This doesn't seem to add anything to the existing answers.

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text:
Does air traffic control identify a distress situation when an aircraft flies triangles in the sky?
There is an account in a TSB Canada investigation report after a loss of radio and transponder:

The emergency procedure to be followed by a pilot to alert radar
stations when in distress and unable to establish radio contact is to
fly a left-hand triangular pattern twice with two-minute legs, resume
course and repeat the procedure at 20-minute intervals.

This story took place in 1998: Is this signal still used today? and has the same meaning in all ICAO countries?

Usually in the US for a towered field, you squawk 7600 for a communications loss and then watch for light-gun signals. For an un-towered field you just keep vigilant about traffic and enter the pattern to land.

I've never heard of it (and it isn't in the FAA's ATC orders) and it seems unlikely: how would ATC know that the aircraft isn't doing a training exercise or just flying in a triangle because the pilot feels like it? ATC might ask an aircraft to make turns to acknowledge instructions if the aircraft can receive but not transmit, perhaps that was what your informant was thinking about? See this question.

Flying in triangles as a distress signal is also mentioned as a plot point in Frederick Forsyth's delightful story The Shepherd (as told in audio every year by the CBC), in which a 1950s RAF pilot just wants to fly his de Havilland Vampire home for Christmas.

@Pondlife Well, assuming it's not common for pilots to fly triangles for fun or training, a reasonable procedure would be for ATC to just ask! If they don't get a response, radio failure is a good assumption.

@DavidRicherby That assumes the aircraft is already in contact with ATC, which may not be the case. It seems to me that this procedure would only be useful in a very limited case: VFR, on flight following (or otherwise in contact with ATC), in radar contact, within scrambling distance of aircraft that could help. If any of those things aren't applicable then either ATC will have no clue that help is needed or won't be able to do anything anyway. And why would a lost pilot waste time and fuel flying around in triangles rather than start navigating and/or looking for a good landing spot?

@Pondlife: Why? Because they are lost, over the sea, it's night and cloudy?... and the compass is broken... or the compass is ok, but they are short of fuel and decided to ditch the aircraft?

@mins That was a rhetorical question, but I see your point :-) I suppose the procedure might help if you can still receive, because you can listen on 121.5 for instructions. But I'd hate to be up there burning fuel with no guarantee that anyone will a) notice and b) be able to help. Of course, maybe in the UK this a well-known procedure and the RAF is able and willing to very rapidly intercept and help lost aircraft; I have no idea.

If I had a complete electrical failure, flying 12 minutes in useless directions will be the absolute last thing on my list.  Even if ATC notices my pattern, what can they do, other than clear traffic in front of me?  I'd just rely on BST (Big Sky Theory), and Land ASAP.

The only conceivable case where this would be at all useful, is if needing vectors to descend through cloud to an airport, after loss of all navigational equipment and possibly all gyro equipment--   not to mention, obviously, all ability to transmit on radio-- yet can still receive--

@quietflyer, Yes, and if radio is completely failed asking for help from an interceptor. Possibly useful when radio failure was more frequent.

To my surprise, this does exist, at least in the UK. According to the Manual of Air Traffic Services:

7 Emergency Triangle Procedure
7.1 Pilots lost or uncertain of position and experiencing either transmitter or complete radio failure are advised, as a last resort, to carry out special procedures to indicate to controllers that they
  require assistance.
7.2 The aircraft fly at least two triangular patterns, before resuming course, as follows:
Table 1:

7.3 If a triangular pattern is observed, controllers shall advise the D&D cell of the position and track and continue to plot the aircraft
  whilst it is within ATS surveillance system cover. Controllers should
  also be aware of the effect wind may have on the pilot’s ability to
  fly an accurate triangle.

I say "to my surprise" because there are well-established procedures for comms failure under IFR and under VFR. But having said that, it's possible that the triangle procedure is more useful in the UK (for example) than the US, where there are usually plenty of uncontrolled airports around and many pilots have no need to talk to ATC anyway. In the UK, with a much smaller airspace, fewer airports and more ATC control (at least, that's my understanding), it may be more critical to give even VFR aircraft a "last resort" to contact ATC.

That does seem a little surprising. It does specify that the procedure is for if you are lost. If you are lost and have total com failure though I'm not sure what ATC can do for you anyway.

@TomMcW: Well they could send an interceptor.

@Joshua, if the plane is flying right triangle, they will first try issuing vectors on the likely frequencies (frequency charted for the sector, last frequency the plane was heard on and 121.5). For one flying left triangle, or one that continues the right triangle after the vectors were issued, sending an escort fighter seems like the only option.

@TomMcW, well, it is not that surprising. It is for the very rare case when the plane can't use the usual lost comm procedure.

@JanHudec A very rare situation indeed. And then hope the ATC is familiar with the procedure. "N12345,  Uh... What the **** are you doing?"  Is sending an interceptor something they would actually do if a pilot is lost?

@TomMcW, for IFR flights, in Europe, one missed hand-over might be enough to get the interceptors up. There seems to be an incident or two every year. But I don't know how easily a VFR flight would raise the controller's attention.

In the UK, the pilot would probably also be more likely to be close enough to controlled airspace that someone will care what pattern he's flying than in the U.S., where they could be a hundred miles or more from the nearest controlled airspace (below 18,000 ft., at least.)

I was also taught this during my VFR training in Australia, two clockwise triangles indicate you can receive but not transmit while two anti-clockwise triangles indicates you can't do either. It's only military, although I'm not really sure it would matter...you approach any controlled airspace without squawking or responding to radio contact and they're gonna figure out pretty quick that something's up.

"it's possible that the triangle procedure is more useful in the UK (for example) than the US, where there are usually plenty of uncontrolled airports": The incident mentioned in the question took place in Canada, but to add to your opinion, it was near Montréal, an area indeed highly monitored.

There is no standard pattern for an emergency(see Pondlife's answer), I suppose if you were really bored you could spell out "HELP" on somebodies radar scope, but if you are operating VFR in the US a controller really isn't going to notice that you are flying a triangular pattern. This would be dangerous though around airfields, since you want to remain in the standard pattern so you aren't confusing or surprising other pilots by coming in from odd angles. The best thing to do in this situation is to fly normally in a manner which other pilots can anticipate.
Squawking 7600 is the code for "Communications Failure" and this is the procedure I was taught for VFR (in the US), although it should hold for any ICAO airspace/field:

Set Transponder to 7600
Say intentions over the radio, the transmit may still work but the receive may be dead
Enter the traffic pattern with a standard pattern entry
Watch the tower for light-gun signals.
Acknowledge Signals by rocking wings or flashing landing/taxi lights.

For an un-towered field, you enter the pattern as normal and follow the traffic flow in to land, taking care to watch for other aircraft on a straight-in or other odd approach.
Class-B offers other issues, if you are outside of class-B and lose your radio you cannot enter the class-B airspace even squawking 7600. If you are already in class-B then you may be able to continue but personally if the aircraft is otherwise flyable I'd head for the nearest class-D or C. There may be some exceptions to this for IFR flight plans, but that should be spelled out in the information in the link below.
Here is a good read of the Radio Loss Procedures and exceptions to the ICAO standard procedures for different operational areas below it.
By the way, you should be flying with a current AFD, and using your cell-phone to contact the tower is a perfectly acceptable way of establishing communications.

you cannot enter the class-B airspace even squawking 7600 I always thought the proper procedure was to squawk 7600, fly the last assigned heading/altitude then follow your flight plan. Does that not apply if your flight plan takes you into class B?

I don't believe it does on a VFR flight plan, IFR may be different but since you are not technically under constant control by an ATC facility, you need to establish 2 way communications and have authorization to enter. If you can't get authorization, you can't enter.

That makes sense on vfr. ATC generally doesn't have your filght plan. On IFR it seems I've read on here numerous times that you simply do what is expected from your plan. I can't imagine they'd want you to stay out of class B because then you'd be off plan and they wouldn't know what to expect.

I suppose if you were really bored you could spell out "HELP" or, if the wind is favorable, "Where Am I?".

This method of flying a triangle pattern to alert air traffic control, or in this case air defence radar control, in case of radio failure, is depicted in the 1955 short film "24 Hour Alert", starring Jack Webb of "Dragnet" fame. Some nice shots of what appears to be March AFB at the time, and F-86Ds and a T-33 in flight.

Welcome to Aviation.SE! While this is an interesting comment, it does not really answer the question (does ATC notice this as distress?).

So, when the Russians, Chinese, whoever, come attacking, they'll fly some triangles and thus be immune from interception?   Hmmm, would this work-- sounds like the basis for a new ASE question--


text:
Why don't short-haul regional jets go for composites?
We have seen Airbus and Boeing going all the way for composites on their A350 and 787, but when it comes to smaller planes like the Embraer E-jets or the Bombardier C-series, they choose to use aluminium, I'd like to know what are the reasons for this to happen, why is more cost-effective?

There are some all-composite business jets, like the newest Bombardier Learjet. The design choice is about trade-offs in manufacturing: supply chain, number to be built, available tools and workers, legal certification (old methods are considered proven so require far less testing data), existing techniques, cost/percentage of build flaws, market price of the end product, needed performance and lifespan.
Then there is a question of available qualified and certified after market mechanics for repair, inspection and maintenance. Downtime is required to repair minor damage like bird strikes and examine internal damage from loose carts or cargo that is a visible dent on metal but only looks like a scuff on composite but could be internal delamination. (Boeing had to consider and develop solutions for all of these external demands. Some solutions only made sense for large carriers with major maintenance divisions.)
For example, if a significant mistake is made on a lamination layer, the whole composite part is pretty much landfill garbage, while an aluminum panel or rib can be recycled and replaced as fairly low cost and effort.
On the other side, business jets like Lear generally have far different flight profiles over their life compared to commercial jets, operate at different cabin pressure, are more concerned with altitude and speed, may consider small airports a constraint, and pennies per seat mile is a much lower consideration. Cost of downtime for repair or inspection is a very different priority as well.
Then there are the aspects of fire, repainting (paint stripping specification), effect of fuel any that may leak into the structure, and of course weather certifications for lightning and icing conditions. For de-icing and anti-icing, aluminum can be easily warmed from the inside while plastic is a poor conductor and may need another method. For lightning protection the 787 has a metal cloth for the outer  layer and electrical bonding between all the parts, and this adds weight and thickness; as a percentage, a widebody jet is less affected by the extra material than a narrow body (the circumference doubles but the cross section area is quadrupled). Not all private aircraft are required to have full lightning protection. If they will not be flying near such conditions, maybe just the fuel tanks will have static and lightning protection. Also, useful payload is far less of a concern for business jets than for regional carriers who often sell extra baggage capacity to freight or parcel carriers like UPS/FedEx.

The Lear 85 was a train wreck from the get-go.  Some genius thought that they could build a carbon fuselage using normal commercial layup methods without using an autoclave, like making tennis rackets or motorcycle fairings.  Think of the cost savings.  It was a disaster and they had to start over.  Eventually the project was canned in 2015.

And on the other side is the Honda jet, all composite fuselage and considered quite a desirable plane.

That wasn't the only problem with the Lear 85 program...

The C-Series has a carbon wing box.  The fuselage is aluminum but is a newer fairly exotic aluminum lithium alloy that is a bit lighter than 2024.  The decision to go with aluminum for the fuselage was mostly development and manufacturing cost (a fuselage requires a massive autoclave and it's development is fairly high risk the first time around) and ease of repair (the fuselage takes the brunt of vehicle damage on the ramp, especially on smaller aircraft) compared to carbon.
Cost is probably the number one driver.  They stuck with things like bleed air services, and wisely decided to avoid lithium ion batteries (Boeing had to use L-ion with the level of electric services).  The C-Series is overall a more conservative design than the 787, and it still almost broke the company, which was forced to give the airplane to Airbus for free or go under.
The E-jet is more or less the old 190 with a complete systems state of the art revamp (and I believe a new airfoil) that make it more or less a smaller C-series system wise without the risks of composites development.  An even cheaper choice, and a wise one on Embraer's part.

The C Series has carbon in highly stressed wing skins and spars, so you have to restrict your question to fuselages if you want to be accurate.
While carbon is theoretically lighter for a given load, in fuselages there is a minimum skin thickness able to resist damage from things like runway debris and hail. It turns out that a narrowbody fuselage is way overbuilt when using carbon, so the potential of lighter weight is not realized. The higher cost of carbon rules it out.
A widebody has higher loads from pressurization of the larger diameter fuselage as well as bending from greater length. Such a fuselage can be made lighter than in aluminum as the required strength is well above the minimum gauge needed for damage tolerance.
Widebodies typically fly long segments, where every kilo of takeoff weight is multiplied by the fuel required to complete the flight. The fuel fraction (fuel/takeoff wt) can be as high as 45% so every kilo of airframe weight matters. Short haul regional flights are much less sensitive, with fuel fractions below 20%. Over the life of the aircraft a kilo of airframe weight is worth several thousand dollars to a widebody.
So widebodies are able to realize a weight savings from use of carbon, and have a flight profile that can turn the weight savings into a cost savings. Narrowbodies can't realize a weight savings, and couldn't reduce operating cost enough to defray carbon cost even it they were lighter.

Reducing empty weight is as important for a narrowbody as it is for a widebody. How do you quantify your claims?

@Koyovis Weight is important for anything that flies, that is true. I explained why it matters more on long haul flights (google Breguet range equation) which happen to be conducted by widebodies. Please explain what claim you object to and why.


text:
Gas Turbine engine inlet
I understand that the turbine of the engine runs the compressor and the fan of the engine. If the air flows through the compressor undisturbed, can we avoid having an inlet in the gas turbine engine to reduce drag of an aircraft?

Welcome! What do you mean by "can we avoid having an inlet in the gas turbine engine"? air must enter the engine thru an inlet (edit your question directly to clarify, using the 'edit' link). By the way, air doesn't flow thru the compressor undisturbed, it is compressed, compression is by itself a big disturbance. The cold flow in a turbofan is accelerated, which is also a disturbance.

If there's no inlet, how does the air get... let in?

If you are talking about the inlet (diffuser), this is not a good idea for several reasons
UNIFORMITY OF AIRFLOW THROUGH THE ENGINE
Because of the design of the diffuser, it can eliminate turbulent airflow and, in the case of supersonic aircraft prevent the engine from ingesting shockwaves, causing compressor stalls and flameouts.  Diffusers also direct the air into the compressor in a uniform fashion, reducing engine problems if the aircraft is sideslipped or at high angles of attack.
ADDITIONAL RAM AIR COMPRESSION
The diffuser can provide additional compression at higher speeds vis a vis ram air forced through the diffuser slowing it down and compressing it.  While this process does create additional aerodynamic drag, it provides the engine with denser air to use and subsequently more thrust.
RCS REDUCTION FOR MILITARY APPLICATIONS
Jet engine compressors and fans are strong radar reflectors, which increase the radar cross section (RCS) of an aircraft.  Diffusers can be designed to either hide the engine compressor from view of a radar using a S-shape diffuser or cover the diffuser inlet with a grill having openings narrower than the wavelength of a potential radar beam, much like the grill on a microwave window protects the people around it.

Just a detail: "narrower than the width of a potential radar beam", actually than the wavelength. When this condition is obtained, the mesh acts like a mirror (a reflector).


text:
What does "representative sample engine" mean for an overhaul extension?
If the PT6 turbo prop engine is required for an extension for overhauling due to operational reasons, the manual has to be searched if the extension is feasible for the company. The company states that for the overhaul extension to be carried out a "representative sample engine" is required. What does a representative sample engine mean? Does it specify a part of the engine to be sent to check if the extension is feasible?


text:
GPS Magnetic Declination and current database
Can I fly with an expired GPS database if I verify the waypoints locations? or is the magnetic declination part of the database?

I believe this question has been answered here:  https://aviation.stackexchange.com/questions/11568/can-i-use-an-expired-ifr-certified-gps-with-an-out-of-date-database-in-lieu-of-d

Per your comments about magnetic declination, I don't think that is updated on a yearly basis in the GPS.  The VFR charts show the 2015 value for Isogonic lines.  The actual magnetic declination today is a little different than what the charts depict.


text:
As a General Rule, who is authorized to taxi aircraft
Need Help with this question it is giving me a hard time

What country/jurisdiction? Also where? there may be different regulations for taxiing on taxiways and runways vs maneuvering around a ramp.

As a General Answer, I would say "a pilot". It's hard to be much more specific than that for such a general question.

It's an interesting question, e.g. when the aircraft has to go through maintenance.

Is this a study question for an exam? If so, knowing for what jurisdiction you are asking and for what aircraft type will help immensely. The rules for who may taxi a 747 in Brazil may be different from who may taxi a Grand Caravan in Vietnam. In most real-world cases, the person taxiing the plane is a pilot of the aircraft in question.

Anyone can taxi an aircraft, if you know how.
The only limitations would be those stipulated by the aircraft owner and/or insurance company who would want to set some minimum experience or training so that nothing gets damaged.
Some airports have their own regulations as to who is qualified to taxi an aircraft. These airport regulations might vary depending on the complexity and size of the aircraft.
 PALM SPRINGS INTERNATIONAL AIRPORT
 Chapter 16.32 AIRCRAFT RULES AND REGULATIONS

Only qualified persons shall start and/or operate an Aircraft engine at the Airport and/or taxi an Aircraft on the Airport. Qualified persons include a pilot, an airframe and power plant mechanic, or a qualified technician licensed by the FAA and qualified to start or operate the engine(s) and/or taxi that particular type and class of Aircraft.

Depending on the jurisdiction,  a radio licence might also be required if you need to use the radio.

"A radio licence would also be required if you need to use the radio." - Not in the United States. In the US, no license is required to transmit using an airband radio while you're aboard an aircraft.

Airports frequently have rules who could taxi the aircraft within they boundaries (normally pilot, student pilot or mechanic). This question contains more information.
Moving over ground between two points without intention of flying is not a flight.


text:
Turboprop vs turbofan (in regional aircraft)
I'm trying to figure out WHY turboprops are more efficient at lower altitudes and airspeed, while turbofans are better at faster speeds and higher altitude.
Simplistically, my explanation is the following:

Gas turbine part of both turboprop and turbofan engines is basically the same, so BOTH must benefit from increasing altitude.
However, as TAS also increases with the altitude, turboprops start experiencing supersonic effects (shock waves) at propeller tips, which limits their ceiling.

Is this explanation correct? What formulas can support it? Why turbofan is less efficient than turboprop at lower altitudes?
Thanks.

closely related questions (take a look at the answers):
Why are propeller engines uncommon on airliners? and Why don't airliners have turboprop engines instead of jet engines?. You may hightlight in your question the differences with those ones.

Turboprops are more efficient, because they accelerate more air by less to create the thrust, and therefore the air carries away less energy , since kinetic energy is proportional to square of velocity. This is true at all altitudes.
Neither engine benefits from altitude. They do benefit somewhat from the colder temperatures up high as heat engine efficiency is proportional to the ratio of high and low absolute temperature. But the main benefit from flying high is that drag is proportional to density, so in the less dense air up high the aircraft flies faster with the same thrust. This is true for both engine types.
The key difference is that turboprops are limited by true airspeed as when the propeller tips get close enough to speed of sound, shockwaves start to form that create massive drag (and a lot of noise) and the efficiency declines fast. In contrast the inlet of a turbofan modifies the pressure field around it so that the air always hits the fan the same speed independent of the speed of the aircraft, converting the extra velocity to pressure instead. Therefore turbofans maintain their efficiency to higher true airspeeds.
Since the low density at high altitudes also increases true stall speed, the true airspeed restriction of turboprops creates an altitude restriction too.
So for short routes where there is not enough distance to climb up as high, the higher efficiency of turboprops is better and turboprops are used. But for longer routes where you can climb up above about 30,000 ft and fly most of the route there, the increase in efficiency due to lower density and resulting higher true airspeed offsets the lower efficiency of the turbofan engines and they become better.
Plus flying faster reduces other costs. For short routes the aircraft spends larger fraction of time on the ground loading and unloading and refuelling, so flying faster has less effect and you want to minimise the fuel burn. But for longer routes flying faster means the aircraft can make more trips in a given time and the crews can make more trips and therefore the fixed costs spread over more passenger miles, which makes it worth flying faster even if it means using a bit more fuel. That again requires using turbofan engines.

Thanks for a very detailed reply.

As stage lengths decrease, maximum cruise speed at 39,000 feet plays a diminishing role in determining gate-to-gate durations since there usually isn't enough time to climb to that altitude and then descend again on a commuter hop.
This means that there is a viable flight profile that involves a climb to a lower altitude and a lower speed cruise which comes within spitting distance of the gate-to-gate duration of a faster plane that cruises at a much higher altitude.
This is the domain of turboprops.


text:
Why would a cylinder not work below a certain RPM?
We have an IO 540 Lycoming engine, and cylinder #3 doesn't work below 1900 RPM (cold to the touch; EGT and CHT are zero; no vibration). The engine is new, what could be the cause(s)?
We cannot contact the company.

Hi and welcome to aviation.SE. I'm sorry, but I'm afraid that without looking at (i.e. physically inspect) the engine we can't answer this question.

"The engine is new" So what does the manufacturer have to say about the problem?

We are not a subscriber of the lycoming so we cannot contact the company !!

Where did you obtain the engine?  They'd be the first place I'd contact, if you didn't get it from Lycoming directly.

This is a good question.  Why the downvotes?

Whoever sold you the engine has sold you a defective one. The question of what is wrong is interesting but you should take the matter up with whoever sold you this engine: you paid them for a working engine and they gave you something else.

Well, if it is cold and cht and egt are zero then I you're either not getting spark or fuel or air.
It's possible that the spark plugs are not firing.  Clean and bench test them.  Also check that the mags are timed correctly.   Maybe they work but they just fire at the wrong time.
Make sure the valve train is assembled correctly.  When you installed the cylinder, did the lifters and rods get put back correctly.  Also, are the rockers installed correctly.  I know this sounds like a stupid thing, but sometimes people make errors.
Maybe you have a stuck or broken valve.  You typically notice when this happens though!  Also, if it works at some RPM and not others, this is probably not the issue.
Finally, have your A&P follow Lycoming's fuel system calibration procedure to the letter (you'll need to look this up; TCM's is SID 97-3).  I had a TCM IO-520 on a V35B for a few years and when we calibrated the system we saw a noticable improvement in engine performance.
Of course, check the compressions!  If you're 0/80 then you may have your answer right there.
These are just a few thoughts and it's by no means a comprehensive list of possible causes.

If Cyl #3 is running cold it's not getting fuel, or the fuel it is getting is too lean.  The Bendix RS mechanical injection system used on Lycomings has an idle mode that takes effect when the pneumatic pressure sensing system in the fuel regulator is unable to provide an accurate signal for good operation near idle.
There could be a restriction from dirt or FOD in the line going to Cyl #3 that causes a too lean mixuture in idle mode, but it's ok when off idle mode and the flow/pressure is higher.  Or there is a problem within the regulator itself that is providing insufficient flow to all the cylinders, but #3 happens to be the first to get bad enough to stop working.
In any case, there is not much you can do until you can get an expert on the system to work on it for you.


text:
How are aircraft separated at high altitude?
When cruising at high altitude, a fixed-wing aircraft has less margin for adjusting its speed, due to the convergence of the stall "speed" and the overspeed limit:

If and aircraft has to be separated to prevent it to get close to a preceding, and ATCO asks to the second aircraft to slow down a bit, this may result in speed lower than the stall speed. On the other hand asking it to descend to a lower level may be costly due to fuel rate increasing.
What is the usual way to fulfill such separation need?

Note that an altitude change doesn't have to be permanent - if you're fast enough compared to the aircraft in front that separation is becoming a concern, you're fast enough that you'll overtake it pretty quickly and can move back to your desired altitude

First of all, there is no such thing as a stall speed, but that is a topic of its own.
The ATC system has numerous ways of dealing with keeping aircraft separated, even at high levels. Let me try to summarize some of the most important ones, starting with the planning that happens when planes are still at the ground at their departure airport, working our way towards what happens when they do get (too) close.
Aircraft flying en-route will follow pre-defined routes (known as airways or RNAV-routes). Airways are like highways in the sky, and ensure that air traffic will fly in predictable patterns. This makes it a relatively simple task of predicting potential conflicts (a conflict in this denotes a situation where two or more aircraft get closer than the prescribed separation minima), because every flight will send a flight plan in advance, detailing the planner route, level and speed to fly.
In addition to just specifying lateral paths, airways also have vertical restrictions. This means that, when following a certain airways, you have to be flying in a certain range of vertical levels. It is quite typical for airways to limit traffic to fly at either “even” or “odd” levels. An even level in this case could be 34,000 feet, 36,000 feet, 38,000 feet and so on, while an odd level could be 33,000 feet, 35,000 feet etc. The general rule in most parts of the world is that airways going east will force traffic to fly odd levels, while airways going west will force traffic to fly even levels. This acts as a fall back procedures, which in principle means that two aircraft flying in opposite directions (one heading west, the other east) will never fly at the same level. Of course, it is easy to realise that this rule cannot cover every potential conflict.
We have now looked at some of the permanent infrastructure in the sky, but let’s zoom in further and look at the tactical aspect of flight planning, specifically, air traffic control.
Air traffic controllers, whose job is it to “maintain a safe and expeditious flow of air traffic”, use various means of technology to keep aircraft in the sky separated. In many parts of the world, their job is achieved by processing large amounts of information received in real time about air traffic using radar technology, and communicating with planes using radio. In places where no radar coverage exists, such as over large oceans, only radio is used. If no radar is available, pilots have to periodically call up on the radio and make a detailed position report, stating the location, level, speed and planned route of the aircraft.
Using either radar or pilot position reports, air traffic controllers will predict potential conflicts and try to solve them. (This is really where the essence of your question will be answered).
There are a few different separation minima. For the sake of simplifying the answer slightly, let’s assume we are working with only three:

vertical separation (the vertical space required between two aircraft who pass each other at (almost) the same position
horizontal separation (the horizontal distance required between two aircraft passing each other at the same, or near the same level)
time based separation (the minimum time required between aircraft following the same route, at the same level)

The vertical separation minima is generally 2,000 feet, however, as aircraft altimeters and radars have gotten more accurate, this has been reduced to 1,000 feet in many areas of the world (called RVSM airspace). Considering the fact that most commercial aircraft cruise in a level band between 30,000 feet and 40,000 feet, this gives you 10 different levels to work with (30,000, 31,000, 32,000 and so on). That actually means, that using vertical separation alone, it is possible to have 10 different aircraft at the same position, at the same time, while they are still separated.
Of course, forcing an aircraft to fly at a non-optimum level is not the perfect solution in terms of fuel efficiency, but it can often be the lesser evil (as opposed to forcing speed changes etc.)
Horizontal separation is achieved by (temporarily) changing aircraft routes. This might sound like a big deal, but in fact, potential conflicts are usually detected early enough that only very small adjustments are required. For two aircraft on intersecting tracks, if you detect the conflict between them 10-15 minutes in advance, it will often be enough to instruct either of the aircraft to adjust their heading by as little as 5 degrees to prevent the conflict. Once they are passed, you simply instruct them to return to their planned routes. This will only result in marginally longer flight paths, and so will only cause a small impact on fuel consumption.
Time based separation using speed control is probably the last resort in most cases. Instructing an aircraft to change it speed for a long period can have a serious impact on fuel efficiency. However, there is no need to worry about aircraft being forced to slow down enough to risk a stall (or speed up enough to risk damage to the aircraft). For starters, the pilot would never accept such instructions from air traffic control, and ATC would have to come up with another plan. In addition, air traffic controllers have been trained thoroughly, and are well aware of the performance limitations of different aircraft.
As with heading adjustments however, it might surprise you how little it takes to ensure separation using speed control. This is because most modern airliners actually fly at very similar speeds (usually between 75% and 85% the speed of sound). Changing the speed of an aircraft by just 1% will have a huge impact over a long distance, such as a flight across the North Atlantic Ocean. And so, as with levels, where in principle you have around 10 usable levels, considering 1,000 feet minimum vertical separation, you might also have about 10 different speeds to work with.
Overall, speed control is a very efficient way of maintaining separation that already exists (e.g. two aircraft following each other on the same route, at the same level), because instructing them to fly at the same speed will ensure that the latter aircraft never catches up with the former. Horizontal and vertical separation can be applied when separation needs to be established, for example between aircraft on intersecting or opposite routes.
Finally, let’s briefly discuss what happens if the system fails. That is, if air traffic control, for one reason or the other, fails to provide the required separation between two aircraft.
All modern commercial aircraft are equipped with airborne collision avoidance systems (ACAS), the most common form being the TCAS. TCAS equipment of two different aircraft in proximity to each other communicate directly to determine a) if there is going to be a conflict and b) how to best solve it. I won’t go into details here, but basically, if two aircraft are predicted to get too close, the TCAS systems will work out how to solve the conflict, typically by instructing one aircrew to descend while instructing the other one to climb. The fact that the TCAS systems communicate ensures that the same instruction is not issues to both different aircrews (imagine the airplanes flying toward each other at the same level, both being instructed to climb …). If the TCAS goes off, all instructions received from the TCAS system takes precedence over air traffic control instructions (much like air traffic control instructions take precedence over the general airways rules we covered earlier).
So to sum it all up – aircraft separation comes down to planning. If it is predicted, based on flight plans received from hundreds of different aircraft, that a certain route or level will be very busy for a certain period of time, then the system will simply not allow more aircraft to fly this way. They will be re-routed or their planned level changed, even before they leave the gate (allowing them to bring extra fuel, if required, to fly a slightly longer route or at a non-optimum level). Once in the air, air traffic control will make use of vertical, horizontal and time based separation, and, if all else fails, on-board computer systems will ensure that no collisions actually occur.

Edit:
A few references for further reading:
Mid-Air Collision - On the consequences of losses of separation, and how to prevent them
Conflict Detection - On detecting potential conflicts in the ATC system
ATM Sector Management - On predicting capacity and responding to demand in the ATC infrastructure
Flight Plan Filling - On ATC flight plans and their content
Guidelines for providing Radar Separation - A few example calculations
Airborne Collision Avoidance System (ACAS) - On ACAS, the last resort, when all else fails
Reduced Vertical Separation Minima (RVSM) - On the parts of airspace where the minimum vertical separation is reduced from 2,000 feet to 1,000 feet

Outstanding and thorough answer.  Thank you for taking the time to write all of that.  I just think that maybe with there being so much information presented, I would feel more comfortable endorsing it if there were some references linked here and there so that readers know it isn't partial speculation.

So flying a slightly different heading is usually the non expensive solution to resolve a conflict between two aircraft on the same route. Thanks a lot for explaining the whole range of ATC separation methods.

@MortensenAviation - Thank you for your comment. Considering how large a topic ATC separation procedures is, I can't possibly provide references to cover everything. I have however added a few links to further reading material at the end of my reply, which I hope you will find useful.

@J.Hougaard I wasn't saying I didn't +1 you just FYI.  I understand it's not practical to backup every detail.  I mean more like links to a manual or something.

I think you would improve the quality of the post by reordering the elements, providing first the direct answer to the question, then the alternatives and background. For instance TCAS is not directly related to the question, but is part of the separation overall topic.

"First of all, there is no such thing as a stall speed," My understanding is that the stall speed is the speed below which attempting to maintain level flight will result in a stall.

@PeterGreen But that speed is not fixed

The usual way is to change altitude. Going above or below the other plane. The TCAS/ACAS system (the last automated warning to avoid collisions) will give a ascend or descend command to the pilots. The minimal vertical separation between such passing plane should be 1000 ft. Typically it will be 2000 ft because the altitudes are allocated per direction (odd go one way even go the other).
Also there isn't just one altitude airliners fly at. If 2 planes have to follow the same path they will be assigned different altitudes. It's also possible for the planes to be offset laterally by a dozen miles. ATC will in general never put 2 planes close enough that they could crash. If they did then something is horribly wrong and should be investigated. The rules about how close planes are allowed to be is answer in detail here.
Not to mention when the choice is between a potential crash or higher fuel cost the answer is always more fuel. There is no way anyone can justify saving fuel at the risk of collisions.


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What's the meaning of "straight and level"?
Indeed a very strange question.
My understanding of straight and level is: The aircraft is flying at a constant heading and constant altitude, and maybe at a constant speed, and this has nothing to do with the aircraft attitude. However I was reading this in "Big Red Today":

"If you just keep it straight and level, the chances of surviving are
  better," Ms. [...] said. "But as soon as you start to turn, you put
  yourself at risk of a stall-spin.

Here this is clearly about maintaining the aircraft neutral attitude while landing, so losing altitude, and probably speed.
Is this use of "straight and level" correct? What is the definition of straight and level, if any, regarding heading, attitude, speed, altitude?
Note: Please explain for all parameters mentioned above to remove all ambiguities.

Is this use of "straight and level" correct? What is the definition of
  straight and level, if any, regarding heading, attitude, speed,
  altitude?

In the airplane flying handbook the FAA defines straight and level flight as

Straight-and-level flight is flight in which heading and altitude are
  constantly maintained.

My understanding and the instruction I have received has always lead me to understand that Straight-and-Level is independent of Attitude and Speed. However to maintain straight and level flight at a given speed you will have to put the aircraft in a given attitude. Generally speaking at lower speeds aircrafts have a higher nose up attitude to maintain level flight and a more nose down attitude at high speeds to maintain level flight but this can vary by design and wing mounting.
In this case I would say it was used a bit improperly. The statement should have been more along the lines of "the pilot should have maintained his heading and conducted a straight ahead emergency landing. You can read more on why that is here.
You can refer to the full NTSB report of the accident here for all the details.

I'll quote from Rod Machado's first ground school lesson:

Straight flight means the airplane's nose remains pointed in one
  direction and the wings are parallel to the earth's horizon. Level
  flight means the airplane doesn't gain or lose altitude.

(my emphasis)
Therefore, there are three constraints for "straight and level" flight:

Constant heading (i.e. yawing the plane while keeping wings level doesn't count)
Wings level
Constant altitude (i.e. vertical speed indicator reads zero)

It does not constrain:

Speed
Pitch

I believe the author of the article unintentionally used the term wrong. I bet what he meant is "straight and [wings] level", not "level" in "level flight", which means constant altitude.
The point in the article is that one should not be making turns while in close proximity with the ground, as keeping the wings level will provide you maximum lift against gravity, thereby reducing your chances of hitting the ground, or even if it does, reduce your impact energy by lowering the velocity change in the vertical direction, or let the landing gear absorb the impact before it hits the fuselage.

Like many phrases, the meaning of "straight and level" depends on the context.  Unlike acronyms, or specialized phrases like "surface area of controlled airspace", it's not a phrase that should be considered to be "owned" by the FAA.  In aviation, the vast majority of the time, we use this phrase to mean that we are flying in a straight line at constant altitude, with the wings level, without specifying the exact pitch attitude of the aircraft.  But in the context of a crash landing, it's logical to assume it might something else.  The article would arguably be a little more clear if it said a "straight and level attitude", or said something like that, but the statement as written need not be viewed as erroneous.


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Are there cases of aircraft moved off runway by pushing them with vehicles?
And now something completely different for Summer holidays. Arthur Hailey was a RAF pilot who wrote Airport, a novel which later was adapted as a movie by George Seaton. The action is said taking place at Chicago Lincoln International, a fictitious place comparable to O'Hare, while Illinois is under a severe snow storm.
The suspense revolves around a Boeing 707 stuck at an exit, blocking a long runway. Another 707 needs the runway for an emergency landing.

Source: YT
Bakersfeld, the airport manager, wants to push the stuck plane off the runway with snowplows. Joe Patroni, chief mechanics for TWA wants to save the airplane by moving it by its own power.
Clocks are ticking... airports are closing... fuel is starving... Patroni's cigar has gone out.
Bakersfeld releases the snowplows!

What is today's way to:

Prevent a commercial aircraft to be stuck near the runway.
To quickly remove an airplane from the runway should that be necessary.

Is there actual cases of aircraft removed à la Bakersfeld?

American Airlines has a go-team of about 50 volunteers that recover "stuck" aircraft.

Quite a few related cases here: http://avherald.com/h?search_term=grass&opt=0&dosearch=1&search.x=0&search.y=0

Aircraft are ditched on aircraft carriers during emergencies (notably the Vietnam war during the Fall of Saigon) but a bit outside the spirit of the question

In central Africa, passengers might be employed for moving a stuck airliner, but only for a pushback when no ground equipment is available. Sorry, no link - I could not find the source yet.

Just as a purely practical matter, it would seem much more effective to attach a cable from the snow plow(s) to the nose gear (where tugs normally hook up), and tow the plane out of the way.

I just wanted to add a reason it's implausible (or incompetent) for the airport manager to suggest pushing the plane with plows: Airliners are designed to be pushed or pulled by a connector on their nose gear. Applying force to any other part of the aircraft, and especially force strong enough to push an airliner through snow, could damage the aircraft in a way that makes the situation much worse. Simply pushing the gear with a plow could pop a tire. Putting force on the body or wings of the aircraft could tip it off balance and leave it leaning on a wing with one set of gear in the air.

Pushback trucks are called that for a reason, they push aircraft :)
They could be used to push or pull a stuck airliner like you mention, IF the muck isn't so bad they themselves get stuck trying that is. Most AFAIK aren't exactly designed for off road operations.

You could still use your pushback truck to connect to the plane and then a regular cat or so to pull the pushback truck :)

I was a passenger on a flight into snowy Stockholm. The plane failed to line up with the gate under its own power. The captain came on the PA: "we're going to wait for a tug to sort this out". A short while later: "we're going to wait for a bigger tug..."

The plows could clear the snow in front of the plane so the tug has a patch of clear tarmac to work with. The plows could clear the taxiway ahead of the plane as the tug pulls it.


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What could reasonably be done in case 7,500 airplanes must be landed quickly?
I read 5,000 to 10,000 aircraft are flying at a given time. There are several very unexpected but plausible reasons to land all planes quickly, e.g: a war / terrorism event; a giant volcano eruption); or a major meteorite impact
Whatever, in case there is a sudden need to stop flying, what would happen:

From an ATC standpoint, is there any impossibility to conduct such massive landing?
Is there enough runway throughput to allow planes to land quickly?
Is there enough room to park all aircraft?

Is there any preparation for this kind of operation in some countries?

You could read about the last time this happened: The Day the FAA Stopped the World.

What is "quickly" or "at once" mean? Can aircraft be put in a holding pattern and given an hour to get on the ground? Does it have to be in 10 minutes?

@RonBeyer: I'd say as soon as possible, and I'm interested in what is possible, taking into account possible priorities.

The scenario here is somewhat misleading. There's no single organization that would have to land all the planes, since they're spread all over the world.

@GregHewgill I guess "The Day the FAA Stopped the USA and Canada Stopped Itself" isn't such a catchy title. Is Time aware that there's actually quite a lot of world outside the USA? You know, the place where all those international flights come from.

@GregHewgill During 9/11 the priority was not to get all planes on the ground as soon as possible, though – many incoming planes were turned away from the USA, and diverted (mostly to Canada) or flew back to their origin. So, while related, I think the premise of the current question is somewhat different.

@DavidRicherby: "There's no single organization that would have to land" Agreed, I don't suggest this is the case.

@mins Well, if you accept that it's a bunch of mostly independent organisations doing mostly their own thing, it doesn't seem to make much difference that it's a global event rather than just a regional one.

Assuming that GA/small private and commercial aircraft land at separate locations:
Say that around 80%, of those airplanes are GA flights (which they probably are); you would have around 6000 GA airplane flights based on your average number of 7500. Based on that number, it would be theoretically possible to stack quite a few landings concurrently à la Oshkosh. Given that three runways there can land and park somewhere around 8000 airplanes in a couple days (from 6am to 8pm), I would say that spreading that many planes out across all the airports in the world, you could ground almost all GA aircraft within an hour or so.
Note that this kind of operation requires a bit of set up and coordination, so it would probably take a longer to actually pull this operation off.
Commercial aircraft would take a bit longer though as a good number of them would be flying at cruise altitude from which it takes ~30 minutes to comfortably get down from. However, some of these aircraft would be in an over-water flight which would take maybe 4 hours to even reach a decent airport, after which it would have to get into the pattern which might toss on another hour or so to that.
Under the best conditions, I would say that to clear the skies would take around 4.5 hours, but it could probably take longer than that due to airplane concentration in certain areas and some people messing up their landings.

Flight Aware at the moment (6:15 UTC): "Tracking 8,323 airborne aircraft", I don't think 80% are GA, so your base of 1,500 commercial aircraft is probably low.

Yes, I've been wondering what the number of planes including general aviation is. The number of 10k aircraft airborne seems to be passenger jets. I don't think GA is 80% of what's in the air, certainly not outside the US. A passenger jet is in the air maybe a fifth to two thirds of the time, 2000 to 5000 hours a year. Most GA planes fly only hundreds of hours per year. So, even if the GA fleet is larger than the jet fleet, they're not in the air that much, and as such constitute less than half of the planes in the air, I'd say.

@mins, flight-tracking websites such as FlightAware have effectively 100% coverage of commercial flights, but far lower coverage of general aviation.

There's roundabout 400 paved runways over 2438 m (8000 ft) in the USA and EU, respectively, and about 200 or so in China and Russia, respectively, so that's about 1200. They are long enough to deal with most aircraft. Some might be untowered or unavailable, but then there are other countries, too. Estimates for "commercial" airports worldwide run around 4000 (there are about 9000 IATA codes, apparently).
So, every suitable runway would have to accommodate around 2 to 10 aircraft.
You can have around 50 to 70 movements per runway per hour, though you need separation of up to 3 minutes to avoid wake turbulence if a smaller jet arrives after a heavy. Let's say 30 landings per hour, and we're talking about 20 minutes, just in terms of pure runway capacity.
Now, to get all those planes lined up nicely... Parking might get crowded, too, but I'd think it would be doable - just fill up taxi ways progressively (presumably you don't care about take-offs for a while). See this diagram of aircraft parking prior to the Tenerife disaster (where many planes diverted to the small Tenerife airport due to a bomb in Las Palmas).

At any rate, the practicalities would be daunting. Would be interesting to see a proper feasibility study.

"There's roundabout 400 paved runways over 2438 m [...] You can have around 50 to 70 movements per runway per hour", but some of them may not have the correct QFU, or they intersect reducing the throughput. In practical probably only a part of them will be usable simultaneously, and the throughput could be affected by less easy movements on the ground.


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Does the Boeing 737 MAX use electro-hydraulic actuators?
I know the B747-8 uses electro-hydraulic actuators as does the 787, but what about the 737 MAX?

an electric hydraulic system - what do you mean?

I'm guessing Ethan is asking about the use of electro-hydraulic actuators and I have edited the question accordingly. See also http://www.airliners.net/aviation-forums/tech_ops/read.main/343070/

The Max will have hydraulic systems very similar to the NG airplanes, except for the spoilers; those will be "fly-by-wire" and electrically controlled, but still hydraulically actuated. (Edited to correct what I had wrong in the original post. The comments are correct.)
Discussion of the FBW spoilers' capabilities

Very close, but the spoilers are still hydraulically actuated. The actuators themselves are unchanged from the NG. The valves that allow pressure to reach the actuators are electrically controlled now instead of mechanically. The spoiler mixer referenced in your link controls valves the allow pressure to reach the actuators. Now that the valves are electric, ground and flight spoilers are controlled by the spoiler control electronics also referenced in your link. If the spoilers were electrically actuated, there would be no hydraulics required at all as is the case with 787 brakes.

Electrically commanded servo valves but powered by conventional hydraulic systems with high pressure lines running throughout the aircraft, is what I understand from the article. This is not the same as electrically actuated. The 737 family uses a common type rating, spoiler actuation is a 50 year old, proven design. The A320 fly-by-wire ailerons and elevators are hydraulically actuated..

No. 737 MAX hydraulic power is the same as 737 NG hydraulic power. There are pumps in the engines and wheel well. More info can be found here. http://www.b737.org.uk/hydraulics.htm
Edit: It appears some took my link to be an answer about the NG. It wasn't. It was about the system being unchanged. The spoilers are still actuated hydraulically, but the control valves providing hydraulic pressure to the actuators are electrically controlled now. The command is now electronic, but they are still moved by the same hydraulic actuators they always were.

That's info on the B737NG. How about the spoiler actuators on the 737MAX ?

The actuators are exactly the same as the NG. The control valves the allow pressure to reach the actuators are electrically controlled now, and commanded by spoiler control electronics. Ralph J above is fundamentally incorrect, but the downvote on this answer prevents me from commenting on that answer. If the spoilers were electrically actuated, then that would mean the actuators were electric. If you want to know more about that, you can dig into 787 braking systems. Those are electrically actuated with no hydraulics running to them at all.

That sounds plausible.


text:
Calculations for altimeter settings for variations in barometric pressure
Pls help me with these 2 questions
An aircraft is standing on an airfield 220ft above sea level on a day when the barometric pressure at ground level is 1004 mb. If the pilot sets the altimeter to read 220ft on this day, what will it read if the barometric pressure drops to 992 mb.
An aircraft sets off from airfield A (126 ft above sea level) where the ground pressure is 1010 mb and flies to B (762 ft above sea level) where the ground pressure is 985 mb. If the pilot sets his altimeter (incorrectly) at 26ft at A, what will it read when it lands B?

An online E6B is at http://www.csgnetwork.com/e6bcalc.html.

I don't see how this question is likely to ever help any future visitors.

It looks like these questions come from a written test or a sample test? If so, I suggest that you explain what problem you have in answering them, so that someone can explain how to do the calculations rather than just giving you an answer that you still don't understand.

This question appears to be off-topic because it is asking for the answer to sample knowledge test questions. While this site can be a great learning resource you won't learn much if we just give you the answers. Please reword your question to tell us what part of the related concepts you're unclear on and we'll reopen it so you can get answers that improve your understanding as opposed to just memorizing numbers.

Question 1: 550 ft.
Question 2: 660 ft.
(rounded to the next multiple of 10).
Atmospheric pressure drops by 1 mbar for every 8.5 m or 27.7 ft of altitude between 0 and 1000 ft, and the differences of your cases from the standard atmosphere are small, so I did not use corrections.


text:
Why do airliners on autopilot climb during cruise?
Several years ago I was a passenger on a near cross-country flight when the pilot came on the PA and announced that we were climbing towards our cruise altitude, on autopilot, and as our fuel load was burning off, and the plane's weight lessened, we would be climbing about another five thousand feet from our cruise altitude.
I wondered why the autopilot, instead, would not make micro-adjustments to the angle of attack to maintain the same altitude, rather than waste fuel making an unnecessary climb.
I realize that maintaining the same exact altitude is probably not realistic, but autopilot, I would think, ought to be able to maintain cruise altitude within about one thousand feet.
This was on a Southwest flight, so it had to be a Boeing 737.

Autopilots maintain cruise altitude much more closely than +/-1000ft! Standard vertical separation is 1000ft or 2000ft so planes would crash into each other if they could only maintain altitude to +/-1000ft. If you look at the flight progress information on the seat-back display, you'll see that airliners typically stay within a few feet of the designated altitude, presumably with more deviation during turbulence.

The autopilot does maintain altitude. If they cruise climb it is because it was commanded to climb. It will not drift on its own.

This sounds like a description of a step climb

Climbing further while cruising is saving fuel, not wasting it.
First, every airplane has an optimum angle of attack for most economical cruise. If it loses mass (due to fuel consumed), it needs to adjust either its altitude or its speed to keep that angle of attack. The goal is to keep dynamic pressure, the product of air density and the square of airspeed, constant. Since you want to reach your destination in the shortest time, climbing into less dense air is the better option.
Secondly, gas turbines need less fuel per unit of thrust when flying in colder air. As long as the plane stays in the troposphere, it will fly more economically when it flies in colder air.

They also need less fuel overall to provide thrust in thinner air, and in fact the fuel flow rate has to be dialed back as the aircraft climbs or the turbines will stall from the rich mixture. So to a point, the higher the altitude, the better the fuel economy, until the air is too thin for the engine's thrust to maintain the desired forward airspeed.

@KeithS: Almost - the thrust available from the engines goes down with density, so fuel flow is only dialed back in climb, when they run at maximum power. Note that the rate of climb will also decrease accordingly.

I don't get it - as an aeroplane loses mass, the speed (or rather dynamic pressure) for an optimum angle of attack decreases so it has to slow down (fly at lower IAS) to maintain it. So how it is possible for him to maintain speed while climbing? TAS increase compensates a necessary drop in IAS?

@Konrad You can either reduce speed or density. All you need to keep the lift coefficient constant while mass is declining is to reduce dynamic pressure. Climbing reduces density as required.

Now its obvious… thank you!

Along with engine economy they may also have been climbing to take advantage of winds aloft at that altitude. Lets take a look at todays winds aloft out of JFK as an example

FD1US1
DATA BASED ON 300600Z
VALID 301200Z   FOR USE 0800-1500Z. TEMPS NEG ABV 24000
FT  3000    6000    9000   12000   18000   24000  30000  34000  39000
JFK 2109 2109+10 2614+06 2521+01 2341-10 2348-22 235536 245645 246456

We can see that the winds at 34000 are 240 @ 56 and another 5000 feet up at 3900 are 240 @ 64 a solid 8 knot increase. Over a long distance this can make a big difference.


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Can anyone explain lift before the experiment?
The sensors on the wing tell me lower pressure above the wing, the smoke visualization tells me the air stream is faster when flowing over the curve. But can anyone tell me why they behave like that before the experiments?

For the same reason that the speed of light is 300'000 km/s: this is just how it works the particular universe where we live  Anyway the physical/mathematical explanation of that phenomenon is termed "momentum conservation"

The navier-stokes equations perfectly explain lift, but good luck a) solving them, or b) explaining them to a class of curious fifth graders

Before the experiments? It feels like a lot of context is missing.